Math : I have no idea how to approach this question

http://www.admissionstests.cambridgeasse…

question 15

is it trial and error? can anyone explain to me their approach please?

thanks

There are 6 squares available squares that don't touch any of the other squares in any direction (meaning all squares around them are completely empty). There were two squares on their own, two squares next to each other, and two other squares next to each other. Together there are 13 possible combinations of pairs (because remember that the two squares you pick can't touch each other as well).

Third row: 5th one and 10th one (counting from left to right)
Fourth row: 10th one
Fifth row: 10th one
Seventh row: 3rd one and 4th one
Eighth row: 7th one

Iceman is right. There ARE 7 squares. The possible combinations are:

3-5 with 3-10, 3-5 with 4-10, 3-5 with 5-10, 3-5 with 7-3, 3-5 with 7-4, 3-5 with 8-7

3-10 with 5-10, 3-10 with 7-3, 3-10 with 7-4, 3-10 with 8-7

4-10 with 7-3, 4-10 with 7-4, 4-10 with 8-7

5-10 with 7-3, 5-10 with 7-4, 5-10 with 8-7

7-3 with 8-7

7-4 with 8-7

Answer: 18 combinations of pairs

I just went in order for each combination for each square (but you don't repeat a combination, like 3-10 with 7-4 is the same as 7-4 with 3-10). By the way, by 3-10, I mean Row 3 Square 10, in case that wasn't clear.

Outstanding answer, thank you.

All the best to you both.

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I see 7 squares available, but I'm not sure about how to pair them, maybe "nanabana" can help see if 7 or 6 is correct and pair them if it was 7.

Regards.